A NATION CHALLENGED: KABUL; AFGHANS ROUND UP HUNDREDS IN PLOT AGAINST LEADERS

Afghan security officials said today that they had arrested hundreds of political opponents, thwarting a conspiracy to mount a bombing campaign whose targets were the government of Hamid Karzai and the former king, Mohammad Zahir Shah.

A senior adviser to Mr. Karzai, chairman of the interim Afghan government, said that more than 700 people had been detained in the last 48 hours, and that the plot appeared to have been aimed at toppling the government. Another security official said the plotters had a more general aim of destabilizing Mr. Karzai's regime.

''They were planning to explode bombs in Kabul,'' said Din Muhammad Jurat, director general for security at the Interior Ministry. ''They wanted to complicate conditions for the interim government. We finally decided that we had to arrest them.''

Afghan officials said the conspiracy was linked to Gulbuddin Hekmatyar, a longtime warlord known for his anti-Western views and his ruthlessness on the battlefield.

With details of the plot so sketchy, the fact that the roundup focused on well-known opponents of Mr. Karzai's government seems certain to prompt suspicions that the government fabricated the threat to crush its opponents.

Many of them belong to the Pashtun ethnic group that predominates in the south. Mr. Karzai himself is a Pashtun. But his government is dominated by Tajiks, who formed the core of the resistance to the Taliban.

The government did not specify which of its agencies had made the arrests, but a spokesman for the international security force in Afghanistan said it had not been involved.

Other officials gave smaller numbers of those arrested. Mr. Jurat placed the number at ''more than 200.'' Lt. Col. Neal Peckham, the spokesman for the international force providing security in the capital, said the number was closer to 300.

Many of those arrested are said to be Pashtuns. Colonel Peckham said weapons had been seized.

The wave of arrests illustrates the fragility of the American-backed enterprise here, intended to foster democracy in a country with a long tradition of cutthroat and tyrannical politics.

The Americans are providing money and advisers to Mr. Karzai's government with the hope that he will lead the country toward nationwide elections and a pro-Western, moderately Islamic orientation.

Afghan officials said many of those arrested had traveled recently to Pakistan and to Iran, where Mr. Hekmatyar was last seen. Mr. Hekmatyar, whose rocket attacks on Kabul during the 1990's were blamed for as many 50,000 civilian deaths, was reported to have left Iran in February after vowing to expel the Americans from Afghanistan and topple the Karzai government.

Mr. Hekmatyar does not exert his power from one established base, and his whereabouts are not known. Recent unconfirmed reports have placed him in southwest Afghanistan.

As the Taliban government in Kabul neared collapse last November, Mr. Hekmatyar said he was offering secret arms caches inside Afghanistan to the Taliban. In interviews this year, Mr. Hekmatyar, a Pashtun, has objected to the Tajiks' dominance. When asked last week about Mr. Hekmatyar's whereabouts, Zalmay Khalilzad, President Bush's envoy to Afghanistan, declined to comment.

Among those arrested was Wahidullah Zahabaun, the former finance minster for the Northern Alliance and a former member of Mr. Hekmatyar's Islamic Party, which was known for its extreme religious doctrines and its virulently anti-Western views. A government official said that Mr. Zahabaun had been released but that his whereabouts were unknown.

A spokesman for the American Embassy said tonight that the staff did not know about the arrests.

Mr. Karzai's government, cobbled together during a meeting in Germany while the fighting was still raging in Afghanistan, has been plagued by infighting since it took office.

In February, Abdul Rahman, the civil aviation minister, was killed by a mob, and three members of Mr. Karzai's government, including the deputy intelligence minister, were arrested. Mr. Karzai charged that Mr. Rahman had been assassinated as part of a conspiracy. The three men are awaiting trial.

Last month, Zahir abruptly postponed his scheduled return to the country amid concerns about his security. A Western diplomat said the former king faced the threat of assassination.

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The alleged conspiracy comes two months before the convening of the loya jirga, a planned gathering of the nation's political and religious leaders to choose a new government.

The maneuvering for that convention has already begun, with persistent reports that an alliance of Islamic fundamentalists, including Mr. Hekmatyar and others, would try to unseat Mr. Karzai and form a more strictly Islamist government.

Mr. Karzai could not be reached for comment today, but a senior adviser suggested that he might have had little to do with the arrests, and that the arrests might have been carried out without his approval.

Mr. Karzai's critics see him as a compromise choice, and little more than a puppet of the Tajiks who control the Foreign Affairs, Interior and Defense Ministries.

''This is a deeply divided government,'' said the Karzai adviser, who spoke on condition of anonymity. ''I am not sure that he signed off'' on the arrests.

The arrests follow the decision by the Bush administration last month to oppose the expansion of the 4,500-man international security force now patrolling the streets of Kabul.

Mr. Karzai had urged Western governments to expand the force to other Afghan cities, saying that without a national army, his government was powerless to fight remnants of the Taliban or quash restless warlords.

The administration argued that the nations now supplying troops, like Britain and France, had military commitments elsewhere and were not willing to contribute any more. The Bush administration is cautious about the force, for one reason because it has said it does not want to be put in the position of having to evacuate it should fighting make that necessary.

At a ceremony in Kabul today, the Afghan government marked the graduation of the first 600 members of the national army, a force intended to bring Afghanistan's many ethnic groups together under a unified command.

Also today, the new American ambassador to Afghanistan, Robert P. Finn, presented his credentials to Mr. Karzai at Gulkhana Palace. Mr. Finn is the first American ambassador to serve here since Adolph Dubs was kidnapped and murdered by leftist extremists here in 1979.

While Kabul appears relatively calm under the watchful eyes of the international force, the scene outside of capital is markedly different. The most serious threats have come in the north, where the private armies of Gen. Ostad Atta Muhammad and Gen. Abdul Rashid Dostum, the deputy defense minister, have clashed repeatedly in recent weeks.

Although he has pledged his loyalty to the Karzai government, General Dostum may be preparing to challenge it. A United Nations official and members of the interim government say General Dostum is receiving guns and money from Iran. Gen. Dostum recently invited two former associates of Mr. Hekmatyar to set up operations in the large areas of northern Afghanistan where the general exerts nominal control.

Mr. Hekmatyar rose to prominence in the 1980's as a leader in the American-backed effort to oust the invading forces of the Soviet Union. Despite his extremist views, he received more American money than any other warlord.

After the Soviet Union withdrew in 1989 and civil war engulfed the country, Mr. Hekmatyar's fortunes declined. Despite continued backing from Pakistan, his army stalled outside Kabul, and his forces began a series of rocket attacks on the city that lasted through the mid-1990's. As many as 50,000 civilians were estimated to have been killed.

Mr. Hekmatyar met his match in the Taliban, whose forces defeated his on the battlefield. Mr. Hekmatyar went into exile, but many of his followers joined the Taliban.